In recent years, research and development are advanced rapidly for a high density optical disc system that employs a violet semiconductor laser having a wavelength of about 400 nm, and is capable of conducting recording/reproducing of information. As an example, information of 23-27 GB per one layer can be recorded on an optical disc having a diameter of 12 cm that is the same size as in DVD (NA 0.6, light source wavelength 650 nm and memory capacity 4.7 GB) for an optical disc conducting recording/reproducing of information under specifications of NA 0.85 and light source wavelength 405 nm, namely, the so-called Blu-ray Disc (hereinafter referred to as BD), and, information of 15-20 GB per one layer can be recorded on an optical disc having a diameter of 12 cm for an optical disc conducting recording/reproducing of information under specifications of NA 0.65 and light source wavelength 405 nm, namely, the so-called HD DVD (hereinafter referred to as HD). In the mean time, a protective layer of BD is designed to be thinner than that of DVD (being 0.1 mm for BD, while 0.6 mm for DVD) to reduce an amount of comatic aberration caused by the skew, because comatic aberration caused by the skew of an optical disc is increased, in the case of BD. Hereafter, the optical disc of this kind is called “high density optical disc” in the present specification.
Meanwhile, a plastic lens has advantages that a mass production can be secured while keeping stable precisions at low cost, because injection molding at low temperature (approximately 120° C.) is possible, a long life of a metal mold can be secured and material cost is low. In Japanese patent application publication JP-A No. 2001-324673, therefore, there is suggested an objective optical system that includes a plastic single lens capable of realizing a numerical aperture of NA 0.85 and is used for an optical pickup apparatus.
However, the plastic single lens has a problem that spherical aberration caused by changes of refractive index resulting from temperature fluctuations grows greater, though it has the aforesaid advantages. The reason for this is that changes in spherical aberration caused by refractive index changes resulting from temperature changes grow greater in proportion to the fourth power of the numerical aperture (NA4), and the refractive index changes resulting from temperature changes tend to be more remarkable, in particular, when an optical surface having the greater curvature is formed for realizing a high numerical aperture. Meanwhile, in the following description, the characteristic of an optical element in the case of temperature changes will be sometimes called “temperature characteristic”.
As a technology to correct temperature characteristic of a plastic single lens, International Publication Number WO 02/41307 Pamphlet discloses a technology to correct chromatic aberration and to correct temperature characteristic by providing a diffractive structure and a plurality of step structures extending in the optical axis direction (NPS: non-periodic phase structure) on an optical surface of the single lens. It is possible to improve temperature characteristic, by providing the step structure of this kind on an optical surface.
However, there is sometimes an occasion wherein so-called mode hopping in which the center wavelength skips for several nanometers in a moment in the laser light source used in an optical pickup apparatus, and an apparatus provided with a microscopic step structure such as NPS causes a problem that spherical aberration is increased in accordance with the wavelength slip, in the other words, wavelength characteristics deterioration. Further, a single lens having a high numerical aperture having a microscopic step structure such as NPS on its optical surface causes vignetting of a ray of light, resulting in a problem of a decline of light transmittance.
There is another attempt to improve temperature characteristics to constitute an objective optical system with two lenses. More specifically, the attempt intends to improve temperature characteristics by causing each lens to have refractive power and by loosening a curvature on its optical surface. However, when any optical surface of the two lenses is made to be a flat surface, a light flux emitted from a light source is reflected on the flat surface, resulting in a fear that reflected light is detected by a photodetector to cause a ghost. In contrast to this, when a lens having a flat optical surface is tilted against an optical axis, occurrence of the ghost can be restrained, but a fear of occurrence of comatic aberration is caused in turn.
Further, a problem relating to a working distance is caused when the objective optical system is formed by two lenses. More specifically, temperature characteristics of the objective optical system are improved when the refractive power of the lens closer to the light source is increased. However, it shortens the working distance, which results in a problem relating to mounting on an optical pickup apparatus. On the other hand, the working distance can be secured when the refractive power of the lens closer to the light source is decreased. However, it lessens an effect of improvement of temperature characteristics, which is a problem.